1. Field of the Invention
This invention relates to improvement in a focussing plate for use in a single-lens reflex camera and an 8 mm- or 16 mm-cinematographic camera.
2. Description of the Prior Art
Heretofore, an acryl mat having the same surface structure and diffusion characteristic as those of frosted glass has long been used as a standard focussing plate screen for a single-lens reflex camera, etc.. However, since such acryl mat has no smooth undulating surface, the light quantity to be diffused outside a view-finder is considerable, on account of which, when the lens aperture is constricted, the view-finder sight becomes abruptly dark and granularity of the mat surface (fine granular projections like scattered fine sands) becomes which is disadvantageous.
In order to prevent such granularity from being noticeable, it has been contemplated to render the surface of the focussing plate to be in such fine and uniform structure that no granularity thereon is recognizable, or in a regular structure in view of the granularity deriving from irregular surface structure of the focussing plate.
As the focussing plate which has solved the problem of granularity by using the latter-mentioned regular structure, there is known an overall micro-screen. The overall micro-screen is such one that a multitude of trilateral or quadrilateral pyramids, each having 0.1 mm or so at each side on the bottom surface, are disposed on the entire surface of the screen. This overall micro-screen begins to abruptly darken beyond a certain F-number of the len depending on the angle of inclination of the slant surface in the trilateral or quadrilateral pyramids filled in the screen surface, and becomes of no use for range detection. On account of this, it is used as a replacement screen (a screen which is used exchangeably in conformity to a lens used.)
The overall micro-screen for a standard lens has an angle of inclination of 8 degrees or so for each pyramid (an angle of refraction of upto and including 4 degrees). In this micro-screen, when the lens aperture is constricted, the darkening starts from F-4 or so, and the focussing becomes considerably dull at F-8 or around it. In order to avoid such unfavorable condition, an overall micro-screen having an angle of inclination of 4 degrees or so (an angle of refraction of upto and including 2 degrees) is provided for a small diameter lens. Even in such micro-screen, abrupt darkening takes place at a high F-number. In the case of using overall micro-screens available in general market, some of them produce Moire pattern between it and a Fresnel lens, along with darkening.
There has also been known a screen wherein a plurality of very small lenses are arranged regularly. However, boundaries among these very fine pyramids or lenses are not so smooth that light scattering takes place within a wide angle and, the view-finder to become dark. This result is not desirable for our object.